Tony Buckingham, the founder and chief executive of Heritage Oil, is one of Jersey's richest men who boasts a £500m fortune and is a significant donor to the Conservative Party. But most people will know him as the FTSE 250's most famous former mercenary, a tag he predictably deplores.

The 59-year-old is believed to be a former member of the Special Boat Service who entered the oil business in 1972 as a North Sea diver, before becoming a concession negotiator in the 1980s for Ranger Oil and Premier Oil.

By 1989, the period that would shape Buckingham's public image had begun, when he became an adviser to the government of Angola and assisted its oil ministry in establishing an exploration and production company. Three years later Buckingham had developed such close links that he formed Heritage (then called HOC) to hold oil and gas exploration interests in offshore Angola. Along with Ranger, HOC owned half of a company called ROWAL, which in 1993 was forced to abandon its Angolan oil fields and drilling equipment after part of the country was overrun by UNITA rebels.

To regain the equipment, Buckingham did a deal with former South Africa Apartheid-era operatives Eeben Barlow and Lafras Luitingh, and Simon Mann, the one-time Scots Guard who subsequently gained notoriety for leading the Wonga Coup in Equatorial Guinea. The joint venture that resulted, Executive Outcomes, was a private military company that had initially been formed by Barlow in 1989 and which primarily employed former members of the South African Defence Force and special forces.

Executive Outcomes recovered ROWAL's equipment and the Angola government subsequently engaged the firm to train elements of the army and "support it" against the rebels. The company also signed a similar deal with the government of Sierra Leone in 1995, but by the following year, when Executive Outcomes had become quite high profile, Mann and Buckingham set up an offshoot, Sandline International, with another ex-Scots Guard, Lt-Col Tim Spicer. Famously, Sandline shipped arms to Sierra Leone in contravention of a UN embargo.

Heritage's flotation prospectus states: "Following the cessation of operations and subsequent dissolution of each of Executive Outcomes and Sandline International, there has been no association [by Buckingham] with any private military contractors. Mr Buckingham has had no substantive business contact with Simon Mann since 1998 and no contact of any nature with him since 2000."

He has been busy with Heritage, however. Last month, the firm announced a $20m deal to take over Sahara Oil, a Libyan oil services company based in Benghazi, a deal which followed a private meeting between the foreign secretary, William Hague, and a representative of Heritage Oil. The Libya announcement came after the disappointment of the collapse of a 2009 tie-up that Heritage announced with Genel to explore for oil in Iraqi Kurdistan, although it fell apart because there was no prospect at that time of receiving any cash for its Kurdish exports owing to disputes with the Iraqi oil ministry over the structure of royalty payments. Since the deal collapsed, the Financial Services Authority fined Genel chief Mehmet Sepil a record £1m for insider dealing when he bought Heritage shares after learning before the market that it had made a large oil discovery. Meanwhile, Genel is subsumed into Vallares, the investment vehicle floated by Nathaniel Rothschild and former BP boss Tony Hayward.

In May 2010 Buckingham donated £50,000 to the Conservative Party as well as writing a £5,000 to the Carmarthen West and South Pembrokeshire Conservative Association.

Heritage was embroiled in controversy this year when it was disclosed that the foreign secretary, William Hague, had intervened in a £175m tax dispute between Heritage Oil, its partner Tullow Oil and the Ugandan government.